Article Date: 5/1/2005

imaging
The Ins and Outs of Glaucoma Imaging Devices
BY STUART RICHER, O.D., Ph.D., F.A.A.O., North Chicago, Ill.

The video-/Internet-enabled cell phone, DVD, PDA, IPODs and ever smaller and more powerful and networked entertainment devices and portable computers are transforming our lives. Simultaneously, a transformation is occurring in eye care -- it's the result of the introduction of opto-electronic imaging devices and digital record keeping. Acronyms such as OPTOS, ARIS, RTA, GDx, HRT II and OCT3 are forever altering the way we practice, as well as interact with patients, each other, fellow healthcare providers and the medical insurance industry.

This new technology has raised the bar. Optimal management of eye disorders increasingly requires both structural analysis of ocular tissue and functional evaluation of vision as a result of pathophysiological alterations due to age or disease.

A word of caution

Yet we must purchase and employ imaging technology in a judicious and cost-effective manner to maximize the efficient use of resources in an uncertain economic environment. We must know who and when and where to refer patients for these new, high-tech imaging scans. Which imaging device to purchase depends on your particular practice environment and budget.

After all is said and done, however, there's no substitute for basic skills and fundamental knowledge of ocular anatomy and physiological optics.

On the functional side, we must prepare ourselves to assess retinal potential acuity, blue-yellow and red-green color vision, contrast sensitivity, glare recovery and binocular vision. We must know when to send a patient for electrophysiological testing and laboratory evaluation, particularly when we suspect a hereditary retinal disease. Advanced technology is at best an adjunct to the skilled hand and prepared mind of optometrists educated in the practical use of common clinical and technical methods. There's also no substitute for low-tech compassionate care and empathy.

Can you manage?

In addition to basic skills, and irrespective of choice of imaging technology, the foundation of quality eye care involves detailed knowledge of eye disease management guidelines. For diabetes, this includes the simplified International Classification of Diabetic Retinopathy Disease Severity Scale, the 4:2:1 rule and the International Clinical Diabetic Macular Edema Severity Scale (introduced worldwide in September 2003), as well as knowledge of risk factors for progression and current guidelines for laser treatment.

For age-related macular degeneration (AMD), this includes the AREDS (Age Related Eye Disease Study,) retinal (I-IV) severity staging scale and the ability to characterize and estimate the size of drusen in the retina, visualize hemorrhages, exudates, sensory or retinal pigment epithelium (RPE) detachments, and understand the terms classic, non-classic, foveal, juxtafoveal, parafoveal and occult neovascularization. The O.D. in particular should have a firm grasp of the role that nutritional prophylactic supplementation (vitamins C, E, B carotene and zinc/copper) and lifestyle plays in the prevention of advanced AMD and the possible benefit of the carotenoid lutein (and omega III fatty acids) in preventing advanced disease.

In glaucoma, the practitioner must have a detailed knowledge of the glaucoma trials (such as the Ocular Hypertension Treatment Study [OHTS]) and appreciate the value of early detection of nerve fiber layer loss, followed by structural and functional visual field change. For macular holes, you must understand hole type (classic vs. secondary) and size, progression and staging (i.e., Stages I a/b, II and III/IV) implications for treatment.

At your disposal

Now that we've reviewed the fundamentals for diagnosing and managing eye disease, what are your options in non-invasive, non-contact imaging technology?

RTA (Talia Technology). The RTA (Retinal Thickness Analyzer) provides thickness and topographical analysis, 3-D optic disc topography, retinal nerve fiber layer measurement and optical cross sections, as well as digital fundus imaging. A helium-neon laser slit is projected upon the retina at an angle and a charge-coupled device (CCD) camera for measurements of retinal thickness. It requires a minimum of 6 mm of pupil dilation. The beam is split into two parts with the RPE reflecting one beam and the internal limiting membrane reflecting the other.

The time difference between these two reflections determines retinal thickness. You can determine retinal nerve fiber layer thickness by measuring retinal thickness in the macula corresponding to ganglion cell thickness. Optical sectioning can reveal subtle retinal pathologies such as semi-occult pericentral degeneration or elevation from cystoid macula edema. The RTA measures the overall thickness of nine retinal layers (not the RPE) at once, and this overall retinal thickness image can't yet be separated into its component retinal layers. New viewer software allows you to transmit RTA examinations to other specialists.

GDx VCC (Laser Diagnostic Technologies/Zeiss Meditec). This instrument uses a scanning laser polarimeter to do one thing: look at the integrity of the intercellular structures of the retinal nerve fiber layer. It's a highly portable device that you can use through an undilated pupil. The GDx provides a fundus overlay of circumferential color-coded nerve fiber layer thickness data, which may be confirmatory to a visual field, or even better, precede visual field defect(s) or optic nerve structural tissue alteration. It can also reveal a non-glaucomatous optic neuropathy. The change-over-time feature also provides serial analysis for disease severity and progression. This appears to be a great screening instrument for optic nerve disease (i.e., detection of glaucoma or non-glaucomatous optic neuropathy).


Fig. 1: R/L eye is combined single-page HRT II topographic printout.

HRT II (Heidelberg Engineering). The Heidelberg Retinal Tomograph (HRT) device is a confocal laser scanning ophthalmoscope that measures tissue height relative to a reference plane by analyzing 64 two-dimensional images at different focal planes and constructing a 3-D topographic model of the retina and optic nerve. Its strength lies in monitoring structural progression in glaucoma, with more than 350 papers in the world literature to support its efficacy. This is an excellent instrument for an O.D. who has a busy glaucoma practice.

To measure progression, you need a baseline and at least two follow-up optic nerve examinations. For retinal imaging, Heidelberg has introduced the Macular Edema Module. The major retinal optical reflex arises close to the internal limiting membrane. Measurement of the maximum location of the confocal intensity profile at each image point (x, y) results in a topography image. If retinal edema is present, then the amount of scatter from within the tissue increases, changing the confocal intensity profile width. The topography image of the macula is color-coded and retinal edema is measured in arbitrary units.

As with optic nerve head drusen, a limitation of this technology is that hemorrhage, exudates, lesions of the RPE and macular pigment can all disrupt the signal width measurement. Nonetheless, the ability to measure change over time in both glaucoma patients and diabetics who have macula edema, in the same instrument, is impressive. You can network the images into a medical records system.

Stratus OCT3 (Zeiss Meditec). Optical Coherence Tomography uses cross-sectional images of the retina produced using optical back-scattering of light from a low coherence (white light) interferometer in a fashion analogous to B-scan ultrasonography. OCT computes tomographic images based on the amount of incident light reflected for a given tissue and therefore detailed images of retinal pathology. The anatomic layers within the retina can be differentiated in vivo, providing better anatomic perspective, with a resolution down to 10 µm. You can also visualize true retinal thickness, optic head cross sectional images and circumferential nerve fiber layer thickness. The latter is important in both the early diagnosis of glaucoma via the Stratus RNFL normative database and the early differential diagnosis of non-glaucomatous optic neuropathy.

This instrument has redefined our understanding of the pathogenesis of vitreo-macular disease/macular hole formation (see Figure 2), provided new perspectives in the diagnosis and management of macular edema resulting form either retinal vascular disease or cataract surgery and assisted in the detection of choroidal neovasculization. The scans are quick and efficient, but a dilated pupil and minimal media opacification (i.e., cataract) often improve image quality.

Panoramic200 (Optos). This scanning laser ophthalmoscope uses a dual scanning red and green laser in a novel virtual image optical system, so it's now possible to view an ultrawide 200 degrees of retina, often with an undilated pupil, compared to the 45-degree view possible with a traditional retinal camera. That's about 80% of the retina viewed in one brief shot, which is revolutionary. The image produced is like having two color transparencies laid on top of each other to form the final image. The huge field of view and color separation allows you to distinguish and screen far more pathology (such as multiple microaneurysms in diabetics, semi-pigmented lesions of Gardner's Syndrome and peripheral retinal lesions).

You can store the digital image and transfer it using telemedicine. This isn't going to be your primary imaging device for viewing the clinical macula in detail or the optic nerve in glaucoma/optic atrophy as the Optos contrast settings require you to fine tune them for this purpose. Also, the instrument currently lacks sufficient magnification or stereopsis, a normative database or change-over-time feature. And the instrument can't do scleral depression to eval- uate lesions in the far peripheral retina. However, it appears to be a great screening instrument for evaluating most of the retina.


Figure 2: This patient's Stratus OCT retinal thickness examination OD reveals a stage II macular hole.

Automated Retinal Imaging System (ARIS, Visual Pathways Inc.). This PC-based device employs pupillary tracking, stereo image acquisition and auto-mosaic assembly to create different spectral images that in composite are approximately 70 degrees in field of view. An infrared (IR) flash is followed by a red flash and a green flash; each image type is then combined on a computer to form a full-color image.

The IR flash or red flash provides images of the choroid while the green flash provides images of features on the surface of the retina. Seven 30-degree images are required to compose one single 70-degree montage, which takes approximately five minutes to acquire. The pupil usually requires dilation to 4 mm, but unlike the Optos Panoramic200, stereo base images enable the practitioner to analyze the retina and optic nerve in 3-D using specially designed glasses.

Last but not least -- coding

The reimbursement code for performing a retinal imaging scan is CPT 92135, with no differences in coding between technologies. The code can be applied to diagnosing a retinal disorder, glaucoma or glaucoma suspects including ocular hypertension. The reimbursement is $65 per eye and CMS allows an O.D. to bill once each year.

They're worth your while

Retinal imaging is here to stay. The present ability of these six new technologies to enhance our screening capabilities and to detect change is under study, but the preliminary data are promising. The Panoptic200 and GDx in particular appear to be well-suited for high-volume practices and ocular health screenings of the retina and optic nerve, respectively. The remaining four instruments (ARIS, RTA, HRTII and Stratus OCT) seem more useful in eye clinics, multiple-practitioner offices, hospitals or specialty retinal or glaucoma practices.

I encourage those who have a keen interest in glaucoma to join the Optometric Glaucoma Society; for those interested in the retina, there's the Optometric Retinal Society. At both society's annual meetings, members discuss the pros and cons of all of these technologies in detail.

At the Department of Veterans' Affairs Medical Center in North Chicago, we're fortunate to have both an HRT II and Stratus OCT and are often able to examine patients who are non-communicative because of dementia, stroke or schizophrenia. In addition to showing us things that we haven't seen before, these instruments provide fast and accurate diagnoses, additional information beyond visual fields, fluorescein angiography or B-scan ultrasonography; they often guide our decision to treat or not treat the patient, help us avoid unnecessary testing and, most importantly, help us answer the questions "Does this veteran have glaucoma" or "Why can't this veteran see?"

We're working on integrating the digital data from these imaging instruments into our medical records system. In addition, the instrument displays are great visual aids in explaining a specific condition to the patient and helping him understand the underlying reason for our treatment recommendations. And with greater understanding comes greater compliance, no matter where you practice.

 

Patient Case Files

Figure is of a 79-year-old male patient who has coronary artery disease, degenerative joint disease, atrial fibrillation, benign prostatic hyperplasia, dypsnea, morbid obesity (BMI > 30) and an ocular diagnosis of atrophic age-related macular degeneration OS and low-tension glaucoma OU based on HRTII and corresponding visual field defects. His diet lacks any fruits or vegetables. I've been treating him with travoprost 0.004% (Travatan) OU q.h.s. since September 2003, but on his third serial HRTII scan, I noted clear evidence of progression in each eye and added dorzolamide hydrochloride ophthalmic solution (Trusopt) t.i.d. OU in an attempt to bring the IOP below 12 mmHg OU. I also arranged for a second medical center dietary consultation because of lack of compliance.

An 83 year-old-male patient with HTN and hypercholesterolnemia, reports for an eye examination with complaint of stable decreased vision in the right eye for a period of three years (see Figure 2). He wants to know if a cataract is reducing his vision. Best-refracted acuity is 20/40 right eye and 20/25 left eye.

The patient has normotensive IOP, negative amsler grid, clear lenses and normal-appearing, equally symmetrical optic nerves with 0.3 c/d ratio, but he has a small retinal hole with a cuff about approximately 125u in diameter (the width of a retinal venule) visualized by 90D stereo-biomicroscopy. The Watke sign is negative. The Stratus OCT retinal thickness examination of the right eye, however, revealed a stage II macular hole. I triaged the patient to a retinal specialist for evaluation and potential treatment.

Dr. Richer is chief of Optometry at the Department of Veterans' Affairs Medical Center and is associate professor of Family & Preventive Medicine at Rosalind Franklin University of Medicine & Science. He is an associate professor of Optometry at Illinois College of Optometry and at University of Missouri-St. Louis College of Optometry.

 



Optometric Management, Issue: May 2005